What is it about?

A prominent pattern of variability in the Indian Ocean is a seesaw in sea surface temperature (SST) between the eastern and western sides of the Ocean basin, called the Indian Ocean Dipole (IOD). Its influence on the regional weather and climate is not yet fully established, but the extremely strong IOD event in 2019 provided us the opportunity to consider its impact on the Indian Summer Monsoon. By simulating the response to the anomalous SST patterns that occurred in 2019, and by observation‐based analyses, we find evidence that the IOD did influence the monsoon rainfall in 2019, but that SST anomalies in the Pacific Ocean were also important. Our simulations show that the positive IOD was conducive to wetter‐than‐normal conditions throughout and especially at the end of the monsoon season, but that anomalous warmth in the central equatorial Pacific may have contributed to reduced rainfall in June over India.

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Why is it important?

In this study we intend to investigate the dynamical aspects of the relationship between IOD and Indian summer monsoon rainfall with a specific focus on 2019. That year was peculiar in terms of the seasonal evolution of precipitation over India with dry conditions at the beginning of the monsoon season and very wet conditions toward the end of the season. The results from this study help to understand the role of SST anomalies within and outside the Indian Ocean in affecting Indian summer monsoon rainfall intensity and seasonal evolution during extreme IOD events. This is important for improving seasonal predictions of Indian summer monsoon, and our results also highlight that to predict the seasonal evolution of ISM rainfall, Pacific SST anomalies must be considered even when there is an extremely strong IOD.

Perspectives

A strong positive Indian Ocean Dipole (IOD) in 2019 induced weather extremes along the Indian Ocean rim such as Australian bushfires and African floods. Although the IOD was in its positive phase, the Indian summer monsoon rainfall was quite deficient in June and the observed rainfall was 67% of its long term mean. Later, the monsoon gained its strength from July, ending with an anomalous wet September and contributing to above-normal seasonal rainfall. It is always interesting to study what caused this seasonal evolution of rainfall over India during such extreme positive IOD year.

Dr. Satyaban B Ratna
University of East Anglia

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This page is a summary of: The Extreme Positive Indian Ocean Dipole of 2019 and Associated Indian Summer Monsoon Rainfall Response, Geophysical Research Letters, December 2020, American Geophysical Union (AGU), DOI: 10.1029/2020gl091497.
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